1. Field of the Invention
This invention relates to a control system for an internal combustion engine installed on a vehicle, for controlling the engine according to a property of fuel used in the engine.
2. Description of the Related Art
Conventionally, there has been proposed a control system of this kind in e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2005-344557. In this control system, during fuel cut-off operation of the engine, a predetermined amount of fuel is injected into a combustion chamber, and ignition timing of the fuel is calculated based on a heat release amount parameter. Further, a cetane number of the fuel is determined based on ignition delay determined based on the calculated ignition timing.
In the conventional control system, to estimate the cetane value, fuel is injected when the engine is in an operating condition during which it should be subjected to fuel cut-off operation, and hence the fuel is wastefully consumed. Further, in the control system, only when the engine is in fuel cut-off operation, the cetane number is estimated. Therefore, for example, after refueling, if the fuel cut-off operation is not carried for a long time, the cetane number of the fuel cannot be estimated all this while, and hence the engine is controlled using the cetane number determined before refueling. Therefore, if the cetane number is much different between before and after refueling, the engine cannot be properly controlled, and in particular, when the cetane number of refueled fuel is lower, there is a fear that a misfire occurs.